I blow this guy I know every now and then when he really needs it, and he's big 8.5 x 6 but I feel like I'm doing something wrong because it takes him a while to get really hard. He's bi and he likes me so that's not it. Am I doing something wrong? Anyone have any advice for me on sucking?

nothing wrong at least i doubt it.
sometimes when you head into something, if you're not already hard, it's harder to get it up. if a guy is just sucking and tonguing my soft cock because he just dove down there, it'll take me a while to get up, as opposed to him just letting up for a second and letting me get hard in all of the 10 seconds it'll take

Ask him about it, a guy I see normally takes a while to get hard but if I suck on his balls he's hard instantly, some guys like a sloppy wet deepthroating blow others prefer it quite dry. Some guys just need different things to get them going so ask him what you should be doing.